Printers of various types often use removable ribbon cartridges to provide a supply of inked or carbon ribbon between the printing head and the print medium. As printers have become faster and more reliable, the quality and reliability of the ribbon cartridges has become of greater importance. In some cases, especially with high speed printers coupled to automatic paper feeders capable of unassisted operation, ribbon cartridges have become the weak link in the printing system. Thus, any seemingly small improvement in ribbon cartridge design which will increase ribbon cartridge life without sacrificing reliability, consistency in operation, or low cost is a significant advancement in the field.
One method for decreasing the cost of ribbon cartridges, without decreasing quality, is by the use of reusable cartridges. Such cartridges are typically reused by sending them to a central location where they are opened, their various components cleaned and checked and then recharged with a new ribbon. The recycling of the ribbon cartridges can result in a substantial savings to the user compared with disposing of a spent ribbon cartridge. However, the success of recycling depends upon the continued reliability of the cartridges after repeated use. Economics dictates that the initial cost of a recyclable cartridge must be similar to that of a standard disposable cartridge. Therefore, higher quality, but more expensive, materials often cannot be substituted to improve the cartridges' operating characteristics.
Another factor which has made the reliable operation of ribbon cartridges more important is the increased use of inked ribbons. Inked ribbons have the ability to allow multiple overstrikes, as opposed to the single-strike capability of carbon ribbon. With multi-strike ribbon it is very important that the ribbon be advanced accurately so the character impressions are uniform.
One type of commonly used ribbon cartridge includes a ribbon supply spool, a ribbon drive assembly and a ribbon take-up spool. See, for example, U.S. Pat. No. 4,132,485 to Hess. The ribbon passes from the supply spool, out of the cartridge through a ribbon exit, back into the cartridge at a ribbon entrance spaced apart from the ribbon exit, through the ribbon drive and to the take-up spool. A constant torque is applied to the take-up spool by the printer so that it gathers the used ribbon exiting the ribbon metering drive.
This type of prior art cartridge meters the ribbon through the cartridge via the ribbon drive rather than the take-up spool. The ribbon drive typically includes a drive roller, connected to an incrementally advancing drive in the printer, and an idler roller biased against the drive roller. The rollers have serrations or teeth which securely grip the ribbon passing between them.
One problem with many of these prior art cartridges is that a relatively high biasing force must be applied between the drive and idler rollers to insure accurate, positive metering of the ribbon even at very low ribbon advance rates. These relative high forces dictate that the drive within the printer be sufficiently large to overcome them.
The high biasing forces create correspondingly high amounts of friction between and wear on the various components, including the roughened drive surfaces and the various journal type bearing areas within the case. Although the rollers are often made of durable materials, the case, because of expense, is typically made from a less expensive, but also less durable, material such as polystyrene. Wear among the components can result in decreased consistency in performance in the metering drive. This problem of deterioration of performance becomes more pronounced as the multi-strike capability of ribbons is increased. Also, if the cartridges are intended to be reused, excessive deterioration in the component parts must be eliminated before the cartridge can be considered for such repeated use.
U.S. Pat. No. 4,011,933 to Kern discloses a different type of ribbon metering drive for use in a ribbon cartridge. It uses a fixed position drive roller, positioned by an arcuate support rib, and a floating idler ring, positioned by a fixed post. The centerline distance between the post and drive rollers is fixed so as to just pinch the ribbon between them. The patent states that there appears to be a wedging action between the ring and the drive roller caused by the tension force on the tape tending to push the ring towards the drive roller. A problem with this prior art design has been an unacceptable amount of wear between the ring and post and between the drive roller and support rib. As the components wear, the effective distances between them change; this can reduce the accuracy of the ribbon metering. The wear also limits the life of the ribbon cartridge thus reducing its value as a reusable ribbon cartridge.
Thus, it is seen that prior art ribbon cartridges, because of wear in their ribbon drive assemblies, have a limited life and have a tendency to fail to perform consistently over their life.